Helical structures in vertically aligned dust particle chains in a complex plasma.

Self-assembly of structures from vertically aligned, charged dust particle bundles within a glass box placed on the lower, powered electrode of a Gaseous Electronics Conference rf reference cell were produced and examined experimentally. Self-organized formation of one-dimensional vertical chains, two-dimensional zigzag structures, and three-dimensional helical structures of triangular, quadrangular, pentagonal, hexagonal, and heptagonal symmetries are shown to occur. System evolution is shown to progress from a one-dimensional chain structure, through a zigzag transition to a two-dimensional, spindlelike structure, and then to various three-dimensional, helical structures exhibiting multiple symmetries. Stable configurations are found to be dependent upon the system confinement, γ(2)=(ω(0h)/ω(0v))(2) (where ω(0h,v) are the horizontal and vertical dust resonance frequencies), the total number of particles within a bundle, and the rf power. For clusters having fixed numbers of particles, the rf power at which structural phase transitions occur is repeatable and exhibits no observable hysteresis. The critical conditions for these structural phase transitions as well as the basic symmetry exhibited by the one-, two-, and three-dimensional structures that subsequently develop are in good agreement with the theoretically predicted configurations of minimum energy determined employing molecular dynamics simulations for charged dust particles confined in a prolate, spheroidal potential as presented theoretically by Kamimura and Ishihara [Kamimura and Ishihara, Phys. Rev. E 85, 016406 (2012)].